Use of helium/nitrogen gas mixtures for the laser welding of stainless steel pipes

ABSTRACT

Process for welding together the two longitudinal edges of a sheet of austenitic, ferritic or martensitic stainless steel so as to obtain a welded pipe, employing at least one laser beam having a power ranging up to 12 kW, and in which a gas mixture is used that consists of 30% to 80% nitrogen by volume and of helium for the remainder (up to 100%). The stainless steel sheet has a thickness of 0.5 to 4 mm and the pipe obtained has a diameter of 5 mm to 50 cm. The welding is full-penetration welding.

[0001] The present invention relates to the use of gas mixtures formedsolely from helium and nitrogen in a laser welding process, operating ata maximum power of 8 kW, for welding austenitic, martensitic or ferriticstainless steel pipes.

[0002] It has already been proposed to weld tubes, longitudinally orhelically, using a laser beam.

[0003] In fact, laser beam welding is a very high-performance joiningprocess as it makes it possible to obtain, at high speeds, very greatpenetration depths compared with other more conventional processes, suchas plasma welding, MIG (Metal Inert Gas) welding or TIG (Tungsten InertGas) welding.

[0004] This is explained by the high power densities involved whenfocusing the laser beam by one or more mirrors or lenses in the jointplane of the workpieces to be welded, for example power densities thatmay exceed 10⁶ W/cm².

[0005] These high power densities cause considerable vaporization at thesurface of the workpieces which, expanding to the outside, inducesprogressive cratering of the weld pool and results in the formation of anarrow and deep vapour capillary called a keyhole in the thickness ofthe plates, that is to say in the joint plane.

[0006] This capillary allows the energy of the laser beam to be directlydeposited depthwise in the plate, as opposed to the more conventionalwelding processes in which the energy deposition is localized on thesurface.

[0007] In this regard, the following documents may be cited: DE-A-2 713904, DE-A-4 034 745, JP-A-01048692, JP-A-56122690, WO 97/34730,JP-A-01005692, DE-A-4 123 716, JP-A-02030389, U.S. Pat. No. 4,871,897,JP-A-230389, JP-A-62104693, JP-A-15692, JP-A-15693, JP-A-15694,JP-A-220681, JP-A-220682, JP-A-220683, WO-A-88/01553, WO-A-98/14302,DE-A-3 619 513 and DE-A-3 934 920.

[0008] This capillary is formed from a metal vapour/metal vapour plasmamixture, the particular feature of which is that it absorbs the laserbeam and therefore traps the energy within the actual capillary.

[0009] One of the problems with laser welding is the formation of ashielding gas plasma.

[0010] This is because the metal vapour plasma, by seeding the shieldinggas with free electrons, may bring about the appearance of a shieldinggas plasma which is prejudicial to the welding operation.

[0011] The incident laser beam may therefore be greatly disturbed by theshielding gas plasma.

[0012] The interaction of the shielding gas plasma with the laser beammay take various forms but it usually results in an effect whereby theincident laser beam is absorbed and/or diffracted and this may lead to asubstantial reduction in the effective laser power density at thesurface of the target, resulting in a reduction in the penetrationdepth, or even in a loss of coupling between the beam and the materialand therefore a momentary interruption in the welding process.

[0013] The power density threshold at which the plasma appears dependson the ionization potential of the shielding gas used and is inverselyproportional to the square of the wavelength of the laser beam.

[0014] Thus, it is very difficult to weld under pure argon with aCO₂-type laser, whereas this operation may be carried out with very muchless of a problem with a YAG-type laser.

[0015] In general, in CO₂ laser welding, helium is used as shieldinggas, this being a gas with a high ionization potential and making itpossible to prevent the appearance of the shielding gas plasma, and todo so up to a laser power of at least 45 kW.

[0016] However, helium has the drawback of being an expensive gas andmany laser users prefer to use other gases or gas mixtures that are lessexpensive than helium but which would nevertheless limit the appearanceof the shielding gas plasma and therefore obtain welding results similarto those obtained with helium, but at a lower cost.

[0017] Thus, gas mixtures are commercially available that contain argonand helium, for example the gas mixture containing 30% helium by volumeand the rest being argon, sold under the name LASAL™ 2045 by L'AirLiquide™, which make it possible to achieve substantially the sameresults as helium, for CO₂ laser power levels below 5 kW and providedthat the power densities generated are not too high, that is to sayabove about 2000 kW/cm².

[0018] However, the problem that arises with this type of Ar/He mixtureis that it is no longer suitable for higher laser power densities, sincethe threshold at which the shielding gas plasma is created is thenexceeded, thereby preventing a full-penetration weld to be obtained whenwelding a stainless steel pipe.

[0019] Now, when welding a pipe, it is paramount for there to be totalor almost total penetration of the weld in order to avoid any subsequentfracture of the pipe thus welded, during forming operations such asbending or flaring, or during its subsequent use, when the pipe issubjected to various stresses, such as thermal and/or mechanicalstresses, or else when it has to be used to convey corrosive substances.

[0020] In addition, in some applications, the welded pipe must have ahigh pitting corrosion resistance, that is to say a pitting index PIrelevant to the application.

[0021] The index PI is defined by the following formula:

PI=[% Cr]+3.3×[% Mo]+16×[% N]

[0022] where [% Cr], [% Mo] and [% N] denote the proportions by weightof chromium, molybdenum and nitrogen in the weld.

[0023] As will be understood from this formula, the pitting corrosionresistance increases with the chromium content, with the molybdenumcontent, the effect of which on the pitting corrosion resistance is 3.3times greater than in the case of chromium, and with the nitrogencontent, the effect of which is 16 times greater than that of chromium.

[0024] For an application requiring good pitting corrosion resistance ina particular environment, the grade of steel adopted, and therefore itscomposition, depends on the environment.

[0025] When this steel is welded, whatever the welding process,segregation always occurs during solidification of the molten metal, thefirst parts to solidify (dendrite cores) generally containing loweramounts of the alloying elements than the last parts to solidify(interdendritic spaces).

[0026] Moreover, the welding of austenitic stainless steels oftenresults in the molten metal having an austenite+ferrite hybridstructure. The chemical composition of these two phases differ:austenite is richer in “gammagenic” elements (elements promoting thestability of the γ phase (austenite)) whereas ferrite is richer in“alphagenic” elements (elements promoting the stability of the a phase(ferrite)).

[0027] Carbon (C), nitrogen (N) and nickel (Ni) are gammagenic elements,whereas chromium (Cr), molybdenum (Mo) and silicon (Si) are alphagenicelements.

[0028] Thus, in a steel weld having a certain Cr, Ni and Mo content,after solidification, there will be zones that are richer than theaverage composition, enriched with Cr and Mo, and zones enriched withNi, C and N, and therefore zones of different corrosion resistance, someof these having a corrosion resistance necessarily lower than those ofthe base metal.

[0029] A nitrogen enrichment of the molten metal by the use of a gasmixture containing nitrogen makes it possible to reduce, if noteliminate, these differences in corrosion resistance. Nitrogen, beingpredominantly in the austenitic phase, partially or completelycompensates for the lower Cr and Mo content of this phase associatedwith the enrichment of the ferrite with these elements.

[0030] Thus, document JP-A-09220682 recommends a process for the laserwelding of a duplex steel pipe using an He/N₂ mixture, the duplex steelbeing a steel having a generally high pitting index, that is to sayalways greater than 35.

[0031] In view of this, it will be understood that the problem remainsin its entirety for steels other than duplex or superduplex steelshaving a high corrosion resistance, that is to say steels usually havinga lower corrosion resistance.

[0032] The object of the present invention is therefore to provide aprocess for the laser welding of stainless steel pipes, moreparticularly those made of stainless steel of the austenitic, ferriticor martensitic type, with a nitrogen-based welding gas mixture, that canbe employed with a laser power of up to 12 kW, which results in theformation of an effective full-penetration welded joint resistant tocorrosion, particularly pitting corrosion, and to do so despite the factthat the base steel has only a low corrosion resistance, that is to saya pitting index of less than 35.

[0033] The solution of the invention is therefore a process for weldingtogether the two longitudinal edges of a sheet of austenitic, ferriticor martensitic stainless steel, having a thickness of 0.5 to 4 mm, so asto obtain a welded pipe, employing at least one laser beam having apower ranging up to 12 kW, and in which a gas mixture is used thatconsists of 30% to 80% nitrogen by volume and of helium for theremainder (up to 100%) in order to carry out full-penetration or almostfull-penetration welding;

[0034] the stainless steel sheet has a thickness of 1 mm to 3 mm;

[0035] the pipe obtained has a diameter of 5 mm to 50 cm;

[0036] the gas mixture consists of 40% to 70% nitrogen by volume and ofhelium for the remainder (up to 100%), preferably of 45 to 65% nitrogenby volume, more preferably still of 46 to 60% nitrogen;

[0037] the laser is of the CO₂ type;

[0038] a full-penetration weld is produced;

[0039] prior to welding the edges of the metal sheet, the said edges arebrought together into the form of an O-shaped unwelded pre-tube by meansof press rolls;

[0040] the pipe is welded edge to edge with an axial, longitudinal orhelical joint;

[0041] the steel sheet forming the pipe has a pitting index (PI) suchthat:

PI=[% Cr]+3.3×[% Mo]+16×[% N]

[0042] with PI<35

[0043] where [% Cr], [% Mo] and [% N] are the proportions by weight ofchromium, molybdenum and nitrogen in the stainless steel of the sheet tobe welded;

[0044] the focal spot is circular or oblong;

[0045] the gas flow rate is between 5 l/min and 100 l/min;

[0046] the pressure of the gas is between 1 and 5 bar;

[0047] the nozzle delivering the gas is a lateral nozzle having adiameter ranging from 3 to 30 mm or an axial nozzle having a diameterranging from 1 to 50 mm; and

[0048] the metal sheet is moved at a non-zero rate of displacement withrespect to the welding head delivering the laser beam.

[0049] The invention will be more clearly understood from theillustrative examples below and from the appended curves.

EXAMPLES

[0050] The effectiveness of the process of the invention was verified bymeasuring the penetration of the partially penetrated fusion linesproduced with a CO₂ laser focused by a parabolic mirror having a focallength of 200 mm, onto the surface of a metal target made of 304 L-typestainless steel (austenitic steel) and having a thickness of 12 mm.

[0051] The shielding gas consisted of an He/N₂ mixture. The nitrogencontent of the mixture is plotted as a percentage by volume on theX-axis, the remainder of the mixture being helium.

[0052] The gas was delivered to the interaction zone by a lateral nozzlein the form of a cylinder having a diameter of 12 mm and at a flow rateof 24 l/min. The welding speed was 3.5 m/min.

[0053] As may be seen in the figure appended hereto, the penetration ofthe weld beads is at least maintained for high nitrogen contents, thatis to say ranging from 30 to 80% by volume.

[0054] Even a surprising increase in the penetration of the beads ofaround 5 to 10% in some cases is observed.

[0055] In other words, the helium/argon mixtures according to theinvention result in weld bead penetration depths at least equal to thoseobtained with helium, while improving the nature of the back weld bead:wettability, joining fillet, appearance, etc.

[0056] This is because the nature of the back of the weld will be betterwhen the power of the laser passing through the sheet is greater than atleast 5% of the incident laser power.

[0057] However, it appears that, for equivalent welding parameters, thepower of the laser passing through the thickness of the sheet during thewelding process via the capillary or keyhole is higher in the case whena shielding gas formed from a helium/nitrogen mixture is used than inthe case when a shielding gas composed only of helium is used.

[0058] The endothermic dissociation of the nitrogen molecules near thesurface metal plasma plume results in a reduction in the temperature ofthe latter.

[0059] The absorption of the incident laser beam by the metal plasmaplume is then less effective and the amount of laser energy availablebeyond the thickness of the sheet is slightly higher.

[0060] This type of result is reproducible if one of the aboveexperimental parameters is varied.

[0061] The process of the invention is particularly well suited tointegration on an automated line for producing welded pipes, on whichthe forming of the metal sheet (into the form of an O) and itsdisplacement are automated and are accomplished in particular by meansof drive rolls and press rolls.

1. Process for welding together the two longitudinal edges of a sheet ofaustenitic, ferritic or martensitic stainless steel, having a thicknessof 0.5 to 4 mm, so as to obtain a welded pipe, employing at least onelaser beam having a power ranging up to 12 kW, and in which a gasmixture is used that consists of 30% to 80% nitrogen by volume and ofhelium for the remainder (up to 100%) in order to carry outfull-penetration or almost full-penetration welding.
 2. Processaccording to claim 1, characterized in that the stainless steel sheethas a thickness of 1 mm to 3 mm.
 3. Process according to either ofclaims 1 and 2, characterized in that the pipe obtained has a diameterof 5 mm to 50 cm.
 4. Process according to one of claims 1 to 3,characterized in that the gas mixture consists of 40% to 70% nitrogen byvolume and of helium for the remainder (up to 100%), preferably of 45%to 65% nitrogen and of helium for the remainder.
 5. Process according toone of claims 1 to 4, characterized in that the laser is of the CO₂type.
 6. Process according to one of claims 1 to 5, characterized inthat a full-penetration weld is produced.
 7. Process according to one ofclaims 1 to 6, characterized in that the gas mixture consists of 46% to60% nitrogen by volume and of helium for the remainder (up to 100%). 8.Process according to one of claims 1 to 7, characterized in that thesteel sheet forming the pipe has a pitting index (PI) such that: PI=[%Cr]+3.3×[% Mo]+16×[% N] with PI<35 where [% Cr], [% Mo] and [% N] arethe proportions by weight of chromium, molybdenum and nitrogen in thestainless steel of the sheet to be welded.
 9. Process for manufacturinga welded pipe from a sheet of austenitic, ferritic or martensiticstainless steel, the longitudinal edges of which are welded together byimplementing a full-penetration or almost full-penetration weldingprocess according to one of claims 1 to 8.